[Qemu-devel] [PATCH 3/3] rcu: add liburcu knob to configure script

["Emilio G. Cota" <cota@braap.org>]

The default is --disable-liburcu, which means QEMU's RCU
implementation is used.

When building with --enable-liburcu, the memory barrier version
of liburcu (urcu-mb) is linked against. We might want to make this
configurable in the future for greater performance; for now we just
match the liburcu flavor to the one that QEMU's RCU is using.

Note that the rcutorture under test/ doesn't work with --enable-liburcu,
since it relies on some internals of QEMU's RCU. liburcu is tested with
the urcutorture distributed with it.

Signed-off-by: Emilio G. Cota <cota@braap.org>
---
 configure             | 35 +++++++++++++++++++++++++++++++++++
 include/qemu/atomic.h |  4 ++++
 include/qemu/rcu.h    | 38 ++++++++++++++++++++++++--------------
 util/Makefile.objs    |  2 +-
 4 files changed, 64 insertions(+), 15 deletions(-)

diff --git a/configure b/configure
index f185dd0..71c9802 100755
--- a/configure
+++ b/configure
@@ -335,6 +335,7 @@ libssh2=""
 vhdx=""
 quorum=""
 numa=""
+liburcu=""
 
 # parse CC options first
 for opt do
@@ -1129,6 +1130,10 @@ for opt do
   ;;
   --enable-numa) numa="yes"
   ;;
+  --disable-liburcu) liburcu="no"
+  ;;
+  --enable-liburcu) liburcu="yes"
+  ;;
   *)
       echo "ERROR: unknown option $opt"
       echo "Try '$0 --help' for more information"
@@ -1400,6 +1405,8 @@ Advanced options (experts only):
   --enable-quorum          enable quorum block filter support
   --disable-numa           disable libnuma support
   --enable-numa            enable libnuma support
+  --disable-liburcu        disable liburcu, use QEMU RCU instead
+  --enable-liburcu         enable liburcu, the user-space RCU library
 
 NOTE: The object files are built at the place where configure is launched
 EOF
@@ -3277,6 +3284,29 @@ EOF
 fi
 
 ##########################################
+# liburcu probe
+
+if test "$liburcu" != "no" ; then
+  cat > $TMPC << EOF
+char synchronize_rcu_mb();
+int main()
+{
+    return synchronize_rcu_mb ();
+}
+EOF
+
+  if compile_prog "" "-lurcu-mb" ; then
+    liburcu=yes
+    libs_softmmu="-lurcu-mb $libs_softmmu"
+  else
+    if test "$liburcu" = "yes" ; then
+      feature_not_found "liburcu" "install liburcu > v0.6, or --disable-liburcu"
+    fi
+    liburcu=no
+  fi
+fi
+
+##########################################
 # signalfd probe
 signalfd="no"
 cat > $TMPC << EOF
@@ -4370,6 +4400,7 @@ echo "Quorum            $quorum"
 echo "lzo support       $lzo"
 echo "snappy support    $snappy"
 echo "NUMA host support $numa"
+echo "liburcu support   $liburcu"
 
 if test "$sdl_too_old" = "yes"; then
 echo "-> Your SDL version is too old - please upgrade to have SDL support"
@@ -5368,6 +5399,10 @@ if test "$numa" = "yes"; then
   echo "CONFIG_NUMA=y" >> $config_host_mak
 fi
 
+if test "$liburcu" = "yes"; then
+  echo "CONFIG_LIBURCU=y" >> $config_host_mak
+fi
+
 # build tree in object directory in case the source is not in the current directory
 DIRS="tests tests/tcg tests/tcg/cris tests/tcg/lm32 tests/libqos tests/qapi-schema tests/tcg/xtensa tests/qemu-iotests"
 DIRS="$DIRS fsdev"
diff --git a/include/qemu/atomic.h b/include/qemu/atomic.h
index 4795e28..d7ad0d5 100644
--- a/include/qemu/atomic.h
+++ b/include/qemu/atomic.h
@@ -129,6 +129,8 @@
 #define atomic_set(ptr, i)     ((*(__typeof__(*ptr) volatile*) (ptr)) = (i))
 #endif
 
+#ifndef CONFIG_LIBURCU
+
 /**
  * rcu_dereference - reads a RCU-protected pointer to a local variable
  * into a RCU read-side critical section. The pointer can later be safely
@@ -191,6 +193,8 @@
 #endif
 #endif
 
+#endif /* !CONFIG_LIBURCU */
+
 /* These have the same semantics as Java volatile variables.
  * See http://gee.cs.oswego.edu/dl/jmm/cookbook.html:
  * "1. Issue a StoreStore barrier (wmb) before each volatile store."
diff --git a/include/qemu/rcu.h b/include/qemu/rcu.h
index 492d943..0aeb8f0 100644
--- a/include/qemu/rcu.h
+++ b/include/qemu/rcu.h
@@ -40,6 +40,29 @@
 extern "C" {
 #endif
 
+struct rcu_head;
+typedef void RCUCBFunc(struct rcu_head *head);
+
+/* The operands of the minus operator must have the same type,
+ * which must be the one that we specify in the cast.
+ */
+#define call_rcu_first_elem(head, func, field)                           \
+    call_rcu(({                                                          \
+         char __attribute__((unused))                                    \
+            offset_must_be_zero[-offsetof(typeof(*(head)), field)],      \
+            func_type_invalid = (func) - (void (*)(typeof(head)))(func); \
+         &(head)->field;                                                 \
+      }),                                                                \
+      (RCUCBFunc *)(func))
+
+#ifdef CONFIG_LIBURCU
+
+#define _LGPL_SOURCE
+#define RCU_MB
+#include <urcu.h>
+
+#else
+
 /*
  * Important !
  *
@@ -118,9 +141,6 @@ extern void synchronize_rcu(void);
 extern void rcu_register_thread(void);
 extern void rcu_unregister_thread(void);
 
-struct rcu_head;
-typedef void RCUCBFunc(struct rcu_head *head);
-
 struct rcu_head {
     struct rcu_head *next;
     RCUCBFunc *func;
@@ -128,17 +148,7 @@ struct rcu_head {
 
 extern void call_rcu(struct rcu_head *head, RCUCBFunc *func);
 
-/* The operands of the minus operator must have the same type,
- * which must be the one that we specify in the cast.
- */
-#define call_rcu_first_elem(head, func, field)                           \
-    call_rcu(({                                                          \
-         char __attribute__((unused))                                    \
-            offset_must_be_zero[-offsetof(typeof(*(head)), field)],      \
-            func_type_invalid = (func) - (void (*)(typeof(head)))(func); \
-         &(head)->field;                                                 \
-      }),                                                                \
-      (RCUCBFunc *)(func))
+#endif /* CONFIG_LIBURCU */
 
 #ifdef __cplusplus
 }
diff --git a/util/Makefile.objs b/util/Makefile.objs
index ceaba30..eacc4c4 100644
--- a/util/Makefile.objs
+++ b/util/Makefile.objs
@@ -17,4 +17,4 @@ util-obj-y += throttle.o
 util-obj-y += getauxval.o
 util-obj-y += readline.o
 util-obj-y += rfifolock.o
-util-obj-y += rcu.o
+util-obj-$(call lnot,$(CONFIG_LIBURCU)) += rcu.o
-- 
1.8.3